I use a 23 gallon Rubbermaid recycling can with float switches installed to start and stop my RO/DI according to level. The way I installed the switches it must drop 11 gallons before the unit comes on so I have no problems with TDS creep.
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I think I'll have to do some looking locally for some of these they are PERFECTI use a food grade drum. There is a local place that has canned food and they get olives in these 65 gallon drums. They sell them for like $6 each.
Personally, I use two 25 gallon polypcarbonate water storage containers purchased from my local feed store and they came with built in drains. One holds the RO water (for freshwater and plants), that other holds the RO/DI water (for the saltwater);